Transmission systems often use a twisted pair to carry signal information as a difference signal. In such systems, it is useful to generate a mean (i.e., common mode) of the voltages that can vary from rail-to-rail, with a difference signal that can vary independently, but is bounded. The impedance presented to the inputs should be high, e.g., greater than 1 MΩ. Additionally, the speed of response should be high, e.g., much greater than a 10 MHz bandwidth.